1. Field of the Invention
The present invention relates to a magnetic encoder for use in a rotation detecting device for detecting the number of revolutions of one of bearing components rotatable relative to each other and also to a wheel support bearing assembly utilizing such magnetic encoder. The magnetic encoder to which the present invention pertains is suitably incorporated in a bearing seal assembly mounted on the rotation detecting device of a kind utilized in association with an automobile anti-skid brake system for detecting the number of revolutions of at least one of front and rear wheels of an automotive vehicle.
2. Description of the Related Art
A rotation detecting device for use in association with an automobile anti-skid brake system for minimizing the hazard of an automotive vehicle being skidded on a road surface has hitherto been assembled in a number of structures. In general, the conventional rotation detecting device includes a serrated rotor and a detecting sensor, which are arranged spaced a distance from each other by means of a sealing device used to seal the bearing assembly, but are functionally integrated together to define a single and independent rotation detecting device. This conventional rotation detecting device is of a structure capable of sensing the number of revolutions of the serrated rotor, mounted on a rotary shaft, by utilizing a rotation detecting sensor secured to a vehicle knuckle, and the wheel support bearing assembly used in association with such rotation detecting device is protected by the sealing device, disposed independently and laterally of the bearing assembly, from an undesirable ingress of foreign matters such as dusts and dirt and/or water.
The Japanese Patent No. 2816783 discloses a bearing seal assembly designed to minimize the space for installation of a rotation detecting device and also to dramatically increase the detecting performance. This prior art bearing seal assembly incorporates therein a rotation detecting device for detecting the number of revolutions of a wheel and includes an annular slinger and an elastic member admixed with a powdery magnetic material and bonded by vulcanization to the annular slinger, with a plurality of opposite magnetic poles defined so as to alternate circumferentially of the elastic member.
The Japanese Laid-open Patent Publication No. 6-281018 discloses a built-in coder sealing structure so designed as to reduce the axial dimension of the sealing structure to thereby enhance the sealability between a rotating element and a stationary element and to render it to be easily mounted. According to this patent publication, a gap between the rotating element and the stationary element is sealed, with a rotary disc mounted on the rotating element for rotation together therewith. A multi-pole coder is mounted on the rotary disc and built in the sealing structure. The multi-pole coder used therein is made of an elastomer added with a powdery magnetic material to form a sealing means, with its side face held substantially flush with the stationary element.
The coder made of the plastic material (plastomer) containing a powdery magnetic material or magnetic particles is shaped by the use of a mold assembly having a mold cavity, which is a replica of the shape of a final product, in a manner similar to that accomplished by means of any existing injection molding technique or a compressive molding technique. In other words, the shape of the final product, that is, the coder may be defined by molding it to a shape complemental to the shape of the molding cavity in the mold assembly, or by punching a molded sheet, prepared by the use of a T-die extrusion molding technique or a calendar molding technique, followed by bonding it on a metallic substrate by the use of a bonding agent. In such case, with a metallic substrate incorporated within the molding cavity beforehand, a melt resin may be poured into the molding cavity to allow the article to be simultaneously molded and bonded to the metallic substrate, such as generally practiced in the insert molding process.
The Japanese Laid-open Patent Publication No. 2003-35565 discloses a magnetic member formed by mixing a synthetic resinous paint with a powdery magnetic material.
Any of the foregoing magnetic encoder includes a multipolar magnet containing the powdery magnetic material. Where the prior art magnetic encoder is used in the bearing assembly for an automotive vehicle, and since the automotive bearing assembly is quite often exposed to such a severe environment as containing saline muddy water found on the road surface, a rusting tends to occur in the magnetic encoder when it is used for a prolonged period of time. In particular, where the content of the powdery magnetic material is increased for compactization purpose, the rusting tends to occur easily. In view of this, the Applicant has thought of the possibility of applying an antirust coating to the multipolar magnet of the magnetic encoder, but it has been found difficult to select a suitable antirusting material.
Since the multipolar magnet prepared from the elastomer or plastomer containing the powdery magnetic material has problems, the Applicant of the present invention has suggested, in its Japanese Laid-open. Patent Publication No. 2004-037441, the use of a sintered element formed as the multipolar magnet by sintering a mixture of the magnetic powder and the non-magnetic metallic powder. Even this multipolar magnet requires an antirusting treatment that suits to the characteristics thereof.
Also, in its Japanese Laid-open Patent Publication No. 2004-085534, the Applicant of the present invention has suggested the multipolar magnet formed with an antirusting coating prepared from a clear type high anticorrosive paint. However, the painting of the multipolar magnet with the modified epoxy clear paint by means of a dipping technique or a spraying technique requires the film thickness to be increased to satisfy the requirement of an anticorrosive property for automotive component parts that are exposed to the road surface, resulting in increase of the cost. In addition, this painting requires the use of a mask, resulting in a complicated and time-consuming process step. Furthermore, in order to secure a uniform film thickness and a satisfactory flatness of the coating formed, the latitude of control of the process, for example, of painting and baking during the formation of the coating becomes limited, with the yield of production consequently lowered.
Yet, in a condition in which the multipolar magnet, which is the sintered element, is fixed on a core metal by staking, in order to increase the anticorrosive property between the core metal and the sintered element, dipping of the multipolar magnet and the core metal into a bath containing the modified epoxy clear paint, painting of the sintered element itself or a hole fill-up treatment of the sintered element is often practiced, but it requires a high cost and is not therefore economical.
In view of the foregoing, the Applicant of the present invention has suggested, in its Japanese Patent Application No. 2003-279563, a multipolar magnet made of a sintered element prepared by sintering a powdery mixture of the magnetic powder and the non-magnetic metallic powder, which magnet is fixed on the core metal by staking to form a sintered element/core metal assembly which is subsequently surface treated to have an anticorrosive coating. The surface treatment is in the form of, for example, a cation electrodeposition to form the resinous coating. However, in the case of a magnetic encoder having the above sintered element/core metal assembly applied with the anticorrosive resinous coating, scratches tend to occur during the anticorrosive coating process and/or pinholes tend to be formed in the anticorrosive coating during the baking, with the film thickness reduced at localized portions, accompanied by reduction in the anticorrosive property at those portions.
The foregoing inconveniences occur similarly in the magnetic encoder in which an anticorrosive coating of a clear type is formed on a surface of the multipolar magnet. Specifically, the resinous coating formed by means of a coating technique, a dipping technique, an electrodeposition technique or any other technique can generally have a film thickness not smaller than 15 μm, but in the event of occurrence of the scratches during the painting process and/or the pinholes during the baking process, there is a high possibility that the film thickness at such portions where the scratches and/or pinholes are formed will decrease to a value smaller than 5 μm, accompanied in reduction of the anticorrosive property. The reduction of the anticorrosive property is not welcome since it results in lowering of the magnetic force the multipolar magnet can emanate.
Also, the magnetic encoder, while in the form of the sintered element/core metal assembly, is press-fitted on an outer peripheral surface of a bearing inner race or the like by pushing the multipolar magnet onto the bearing inner race by means of a punch brought into contact with the annular surface of the multipolar magnet. In other words, the magnetic encoder having the antirusting coating formed thereon is press-fitted while being applied a pressure from the punch. It has, however, been found that the inconveniences of the pinholes and/or scratches remain unremoved and the presence of the pinholes and/or scratches in the resinous coating eventually leads to reduction in anticorrosive property, which in turn brings about reduction of the magnetic force the multipolar magnet can generate.